How to Build a Fence With a Laser

Anything built with a laser is automatically cooler. Lasers are so different from any other light source that they are a source of fascination as well as being useful tools. So why not build a fence from a laser? One type of laser fence is used to trigger machinery to shut down if someone puts a hand into a dangerous area. Another recent design for a laser fence was publicized as a way to shoot mosquitoes. But usually a laser fence is the name given to a laser-based perimeter protection system.

Laser
Photodetector
Electronic detection circuit
Mirrors

Instructions

- 1
  Lay out the perimeter and determine the vertical coverage requirements. For example, if you want a simple tripwire-type system then a single laser beam at one height will do the job. If you want more comprehensive coverage to make it difficult for someone to intentionally defeat the system, you'll want the laser to cover several heights.
- 2
  Review the principle of the laser fence so your layout decisions make sense. A laser fence works by sending a beam of light across an open space to a photodetector. As long as the photodetector is receiving light it puts out a voltage. When the beam is interrupted, the signal from the photodetector stops. An electronic signal detects the change.
- 3
  
  Reflected laser beams can cover a long distance, a lot of area, or both.
  
  Determine the range of your laser. You can do this by pointing your laser at a mirror, which you set up to direct the beam to another mirror. By slightly tilting the second mirror, the beam is reflected back to the first mirror, but at a slightly different spot. So the beam is sent back and forth between the mirrors several times. Put the detector in the beam after each round trip until you determine what the maximum number of bounces the laser can make and still be detected. Figure out that distance.
- 4
  Mount the laser to the starting fencepost, pointed to the next fencepost in your desired perimeter. If you want vertical coverage, you will point the beam at a mirror on the next post. That mirror will be tilted up so that the beam returns to the top of the first post, where another mirror is mounted. That mirror will send the beam horizontally out to the third fencepost, where another mirror is mounted, repeating the pattern. This creates a zigzag pattern back and forth between posts. If a single "tripwire' is needed, The laser will be pointed all the way down to the corner post.
- 5
  Turn the beam around corners, if necessary. Mount a mirror to intersect the beam at the corner post and send it in the new direction. To turn a 90-degree corner, the mirror will need to be mounted at a 45-degree angle with respect to the beam.
- 6
  Mount the photodetector. It will go either at the end of the fence, or, if the fence is longer than the distance determined in Step 3, at some distance shorter than the maximum distance. If the fence is not complete, mount a new laser and repeat the steps as necessary to finish your fence layout.
- 7
  Hook up your photodetectors to the detection circuit.

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